Field of Invention
The present invention relates to a power switch control circuit and an open detection method thereof; particularly, it relates to such a power switch control circuit which detects whether an open condition occurs between an operation signal output pin and a power switch, and an open detection method thereof.
Description of Related Art
FIG. 1A shows a schematic diagram of a conventional switching regulator 100A which is an isolated power supply circuit with a transformer. As shown in FIG. 1, a rectifier circuit 101 rectifies an alternating current (AC) voltage Vac to generate an input voltage Vin. The rectifier circuit 101 is for example a bridge rectifier circuit. A primary winding W1 of a transformer 102 of the switching regulator 100A receives the input voltage Vin. A power switch SW1 controls a conduction time of the primary winding W1, whereby the input voltage Vin is converted to an output voltage Vout generated at a secondary winding W2 of the transformer 102. The switching regulator 100A includes the aforementioned transformer 102, the power switch SW1, a power switch control circuit 103, an opto-coupler circuit 104, a current sense circuit 105, and a second side control circuit 107. The power switch SW1 is controlled by the power switch control circuit 103. The power switch control circuit 103 obtains a feedback voltage signal COMP related to the output voltage Vout from the opto-coupler circuit 104, and obtains a current sense signal CS related to a current flowing through the power switch SW1 from the current sense circuit 105, and by feedback control, the power switch control circuit 103 generates an input signal which is typically a pulse width modulation (PWM) signal. (“Input signal” is from the perspective of the output buffer stage.) The power switch control circuit 103 generates an operation signal GATE at an operation signal output pin according to the PWM signal (thus, the operation signal is a PWM signal having a waveform corresponding to the input signal but having adjusted voltage levels), to control the power switch SW1. The secondary side control circuit 107 controls the opto-coupler circuit 104 to generate the feedback voltage signal COMP according to a divided voltage of the output voltage Vout.
Referring to FIG. 1B, in a switching regulator 100B, asynchronous rectification (SR) switch SW2 replaces the diode of the switching regulator 100A at the secondary side, whereby the voltage drop across the diode is removed to increase power conversion efficiency. A power switch control circuit 106 at the secondary side controls the SR switch SW2 through an operation signal output pin, such that when the primary winding W1 is not conductive, the SR switch SW2 is conductive, and the secondary winding W2 releases power, to convert the input voltage Vin to the output voltage Vout.
In the switching regulator 100B shown in FIG. 1B, an open condition may occur between the SR switch SW2 and the operation signal output pin of the power switch control circuit 106 for a certain reason, such that the power switch control circuit 106 cannot control the SR switch SW2 correctly. However, because the SR switch SW2 includes a parasitic diode, when the SR switch SW2 is turned OFF, the secondary side still forms a conductive circuit loop through the parasitic diode. Thus, even though the power switch control circuit 106 does not control the SR switch SW2 correctly, it cannot be detected because the circuit loop at the secondary side is still conductive. This problem may cause the switching regulator 100B to operate incorrectly, lowering the power conversion efficiency, or worse, damaging the switching regulator 100B and leading to a hazardous condition.
In view of the above, the present invention proposes a power switch control circuit and an open detection method thereof, which can detect the open condition between the operation signal output pin and the power switch, so as to increase the power conversion efficiency and to ensure safety. The present invention can be applied not only to the secondary side but also to the primary side of the transformer in an isolated power supply circuit. The present invention also can be applied to other type of switching regulators, to detect whether there is an open circuit between a power switch and its control circuit.